OpenGL学习（五） 着色器渲染（以Android为例）

一、Android平台上下文环境的创建及初始化

1. 首先实例化Android上下文环境，即EGL的初始化。

bool EGLCore::init(EGLContext sharedContext) {
    EGLint numConfigs;
    EGLint width;
    EGLint height;

    const EGLint attribs[] = { EGL_BUFFER_SIZE, 32, EGL_ALPHA_SIZE, 8, EGL_BLUE_SIZE, 8, EGL_GREEN_SIZE, 8, EGL_RED_SIZE, 8, EGL_RENDERABLE_TYPE, EGL_OPENGL_ES2_BIT,
    EGL_SURFACE_TYPE, EGL_WINDOW_BIT, EGL_NONE };
    //eglGetDisplay来返回OpenGL ES渲染的目标，每个厂商都会返回默认的显示设备
    if ((display = eglGetDisplay(EGL_DEFAULT_DISPLAY)) == EGL_NO_DISPLAY) {
        LOGE("eglGetDisplay() returned error %d", eglGetError());
        return false;
    }
    // 初始化显示设备
    if (!eglInitialize(display, 0, 0)) {
        LOGE("eglInitialize() returned error %d", eglGetError());
        return false;
    }
    // 得到配置选项信息
    if (!eglChooseConfig(display, attribs, &config, 1, &numConfigs)) {
        LOGE("eglChooseConfig() returned error %d", eglGetError());
        release();
        return false;
    }
    // 创建OpenGL的上下文环境EGLContext
    EGLint eglContextAttributes[] = { EGL_CONTEXT_CLIENT_VERSION, 2, EGL_NONE };
    if (!(context = eglCreateContext(display, config, NULL == sharedContext ? EGL_NO_CONTEXT : sharedContext, eglContextAttributes))) {
        LOGE("eglCreateContext() returned error %d", eglGetError());
        release();
        return false;
    }

    pfneglPresentationTimeANDROID = (PFNEGLPRESENTATIONTIMEANDROIDPROC)eglGetProcAddress("eglPresentationTimeANDROID");
    if (!pfneglPresentationTimeANDROID) {
        LOGE("eglPresentationTimeANDROID is not available!");
    }

    return true;
}

2. 将EGL和设备的屏幕连接起来。使用EGLSurface，通过EGL库提供的eglCreateWindowSurface可以创建一个可实际显示的Surface，通过EGL库提供的eglCreatePbufferSurface可以创建一个OffScreen的Surface。_window就是通过Java层的Surface对象创建出的ANativeWindow类型的对象，即本地设备屏幕的表示。也就是说真实显示的Surface还是通过Java层创建好的， 然后OpenGL只是绘制到了这个target上边？

EGLSurface EGLCore::createWindowSurface(ANativeWindow* _window) {
    EGLSurface surface = NULL;
    EGLint format;
    if (!eglGetConfigAttrib(display, config, EGL_NATIVE_VISUAL_ID, &format)) {
        LOGE("eglGetConfigAttrib() returned error %d", eglGetError());
        release();
        return surface;
    }
    ANativeWindow_setBuffersGeometry(_window, 0, 0, format);
    if (!(surface = eglCreateWindowSurface(display, config, _window, 0))) {
        LOGE("eglCreateWindowSurface() returned error %d", eglGetError());
    }
    return surface;
}

通过Java层的Surface对象创建ANativeWindow类型的对象方法如下：

JNIEXPORT void JNICALL Java_com_phuket_tour_opengl_renderer_PngPreviewController_setSurface
(JNIEnv * env, jobject obj, jobject surface) {
    if (surface != 0 && NULL != controller) {
        window = ANativeWindow_fromSurface(env, surface);
        LOGI("Got window %p", window);
        controller->setWindow(window);
    } else if (window != 0) {
        LOGI("Releasing window");
        ANativeWindow_release(window);
        window = 0;
    }
}

3. 开发者需要开辟一个新的县城，来执行OpenGL ES的渲染操作，而且还必须为该县城绑定显示设备（surface）与上下文环境（Context）。EGL是双缓冲模式，内部有两个FrameBuffer，当EGL将一个FrameBuffer显示到屏幕上的时候，另外一个FrameBuffer就在后台等待OpenGL ES进行渲染输出了。直到调用函数eglSwapBuffers这条指令的时候，才会把前台的FrameBuffer和后台的FrameBuffer进行交换。

bool EGLCore::makeCurrent(EGLSurface eglSurface) {
    return eglMakeCurrent(display, eglSurface, eglSurface, context);
}

二、Texture/Shader/Program

 1. 创建Texture（纹理）

int PicPreviewTexture::initTexture() {
    glGenTextures(1, &texture);
    glBindTexture(GL_TEXTURE_2D, texture);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
    return 1;
}

2. 创建/初始化/编译Shader

GLuint PicPreviewRender::compileShader(GLenum type, const char *sources) {
    GLint status;
    GLuint shader = glCreateShader(type);
    if (shader == 0 || shader == GL_INVALID_ENUM) {
        LOGI("Failed to create shader %d", type);
        return 0;
    }
    glShaderSource(shader, 1, &sources, NULL);
    glCompileShader(shader);
    glGetShaderiv(shader, GL_COMPILE_STATUS, &status);
    if (status == GL_FALSE) {
        glDeleteShader(shader);
        LOGI("Failed to compile shader : %s\n", sources);
        return 0;
    }
    return shader;
}

3. 创建并使用Program

int PicPreviewRender::useProgram() {
    program = glCreateProgram();
    glAttachShader(program, vertShader);
    glAttachShader(program, fragShader);
    glBindAttribLocation(program, ATTRIBUTE_VERTEX, "position");
    glBindAttribLocation(program, ATTRIBUTE_TEXCOORD, "texcoord");
    glLinkProgram(program);
    GLint status;
    glGetProgramiv(program, GL_LINK_STATUS, &status);
    if (status == GL_FALSE) {
        LOGI("Failed to link program %d", program);
        return -1;
    }
    glUseProgram(program);

    uniformSampler = glGetUniformLocation(program, "yuvTexSampler");

    return 1;
}

 三、渲染操作

void PicPreviewRender::render(){
    glViewport(_backingLeft, _backingTop, _backingWidth, _backingHeight);
    glClearColor(0.0f, 0.0f, 1.0f, 0.0f);
    glClear(GL_COLOR_BUFFER_BIT);
    glEnable(GL_BLEND);
    glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

    glUseProgram(program);
    static const GLfloat _vertices[] = { -1.0f, 1.0f, -1.0f, -1.0f, 1.0f, 1.0f, 1.0f, -1.0f };
    glVertexAttribPointer(ATTRIBUTE_VERTEX, 2, GL_FLOAT, 0, 0, _vertices);
    glEnableVertexAttribArray(ATTRIBUTE_VERTEX);
    static const GLfloat texCoords[] = { 0.0f, 0.0f, 0.0f, 1.0f, 1.0f, 0.0f, 1.0f, 1.0f };
    glVertexAttribPointer(ATTRIBUTE_TEXCOORD, 2, GL_FLOAT, 0, 0, texCoords);
    glEnableVertexAttribArray(ATTRIBUTE_TEXCOORD);
    picPreviewTexture->bindTexture(uniformSampler);
    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
}

执行完渲染操作后，调用eglSwapBuffers即可显示。至此，一次渲染操作完成。